1. Field of the Invention
The present invention relates to a polyester film. Furthermore, the present invention relates to a polyester film for a recording medium preferably used as a polyester film for a magnetic recording medium or a polyester film for an optical recording tape. In particular, the polyester film of the present invention is preferably used as a polyester film for a magnetic recording medium in order to improve a playback output and durability in repetitive playback of a ferromagnetic metal thin film type magnetic recording medium to record digital data for digital video cassette tapes, data storage tapes and the like.
2. Description of the Related Art
In a consumer digital video tape commercialized in 1995, a metallic, magnetic thin film of Co is provided on a base film of 6 to 7 μm thickness by vapor evaporation, and the surface thereof is coated with a diamond-like carbon layer. The consumer digital video tape has a record time of one hour in a standard specification (SD specification) in the case of a camcorder using a DV mini-cassette.
This digital video cassette (DVC) is the world's first consumer digital video cassette, and is valued highly in the market from the following viewpoints.    (1) Enormous amounts of information can be recorded regardless of a small body.    (2) Since signals do not degrade, image quality and sound quality do not degrade even after a lapse of many years.    (3) Since no interference is caused by noises, high image quality and high sound quality can be enjoyed.    (4) Images are not degraded by repetition of dubbing.
Polyester films described below are used as base films of consumer digital video tapes.
(a) A polyester film composed of a polyester film and a discontinuous coating layer which is adhered to at least one-side surface of the film and which primarily contains a polymer blend and fine particles having particle diameters of 50 to 500 angstroms, wherein the discontinuous coating layer contains a water-soluble polyester copolymer, and fine protrusions are provided on the discontinuous coating layer by the fine particles (Japanese Patent Publication of Examined Application No. 63-57238).
(b) A polyester film in which a continuous thin layer containing fine particles having an average particle diameter of 0.1 μm or less is provided as a primer of a magnetic layer by coating on the surface of the polyester film, and the thin layer includes fine protrusions due to the fine particles and fine protrusions-simply due to a binder resin (Japanese Unexamined Patent Application Publication No. 5-287101).
(c) A polyester film in which a continuous thin layer containing fine particles having an average particle diameter of 0.1 μm or less is provided as a primer of a magnetic layer by coating on the surface of the polyester film, and the thin layer includes fine protrusions due to fine particles having an average particle diameter of less than 0.06 μm, large protrusions due to particles having an average particle diameter of 0.06 μm or more and fine protrusions simply due to a binder resin (Japanese Unexamined Patent Application Publication No. 5-298670).
In the fall of 2001, a new video standard was established on a camcorder which was a further miniaturized, light-weight camcorder, which was easily portable, and which was able to be directly connected to the internet without through a computer. This new video standard includes a record time of one hour while the capacity ratio of the size thereof to that of a DV mini-cassette based on the MICRO MV standards is 30%. This video standard is digital recording using the same evaporation tape as that of the DVC. The image compression system of this video standard is not the DV compression based on the DVC standards, but MPEG2 compression. The tape width is changed from 6.35 mm to 3.8 mm, the minimum recording wavelength is changed from 0.49 μm to 0.29 μm, the track pitch is changed to 5 μm from 10 μm of DV and 6.7 μm of DVLP, and therefore, the packing density is significantly increased. A magnetic layer of the evaporation tape matching this new video standard has a significantly reduced film thickness. The Co oxide layer thickness of the MICRO MV tape is significantly reduced to 50 nm compared with the Co oxide layer thickness of the DVC of 160 to 220 nm.
High-packing-density recording and playback can be performed by the new video standard. This is because an MR head (magnetoresistive head) used for a hard disk is adopted for playback.
The MR head takes advantage of the phenomenon in which when a magnetic field is applied to a metal thin layer, the electrical resistance thereof is changed, and the playback output is large. However, the following problems have been made clear.
Regarding the MICRO MV tape prepared from the polyester film in the above-mentioned (a), image defects (dropouts) are very likely to occur. Regarding the MICRO MV tape prepared from the polyester film in the above-mentioned (a), the running life of the MR head is significantly reduced to about 100 hours with respect to continuous playback, and therefore, the MR head must be frequently exchanged.
Regarding the MICRO MV tape prepared from the polyester film in the above-mentioned (b) or (c), image defects (dropouts) are very likely to occur.
Furthermore, when the polyester film in the above-mentioned (c) is used, the MR head must be frequently exchanged approximately every 10 hours with respect to continuous playback.